Don't feel alone because you are not. Trying to figure out all the sizing not only in front but behind your given space can be a project all unto itself. Thankfully there are some resources available to help you out. If stuck or uncertain try using some of these tips.
1. Some kit manufacturers have CAD programs to help you design your layout. Check with your kit manufacturer.
2. You can purchase some computer software to plan your panel. online panel builder
3. You can pay someone to plan your design layout. Larger avionics shops regularly do this kind of work. Aerotronics. Avionkits.
This should get you going in the right direction and when ready let's go to Engine Instruments.